1. Field of the Invention
The present invention relates to a short arc type mercury lamp that is used in the photochemical industrial field, the semiconductor device manufacturing field or the like, and in particular to an electrode of such a lamp.
2. Description of the Related Art
Conventionally, with regard to short arc type lamps, there has been art in which a protruding part is provided on the top of the cathode to suppress arc-wandering of the electrical discharge. This is disclosed in, for example, Japanese Patent Application Laid-open No. 56-134465 (1981).
In the above-mentioned publication, a description is given with regard to a xenon lamp.
A cause of xe2x80x98arc-wanderingxe2x80x99 is that the tip of the cone-shaped part of the cathode, which is pointed when the lamp is first used, becomes to have a spherical surface of large radius as usage time passes due to the tungsten and the electron-emitting substance melting and evaporating, and moreover the structure of the tip part changes, with a tungsten monocrystal being formed and growing, and hence the pointedness is lost, and channel of the electron-emitting substance into the tip part through diffusion is inhibited due to the formation of the monocrystal. In the above-mentioned publication, it is disclosed that by restricting the monocrystal advancement region to a protruding part, the cathode arc-generating point is restricted to the protruding part, which protrudes out from the tip of the cone-shaped part.
With short arc type mercury lamps used in the photochemical industrial field, the semiconductor device manufacturing field and so on, a noble gas and mercury are filled into the arc tube. The coefficients of thermal conductivity of filler substances that may be used in such a discharge lamp are in the order Ar greater than Kr greater than Hg greater than Xe, and hence the thermal conductivity is the lowest in the case of a xenon lamp, for which the filler gas is Xe only.
To keep the discharge arc in the discharge bulb stable for a long time, a certain temperature is required, and in general it is considered appropriate to carry out operation with the temperature at the cathode tip at about 2700xc2x0 C. Here, focussing on the filler gas, if thermal conduction is poor, then the arc will be prone to spreading out, and if thermal conduction is good, then it will be easy to narrow down the arc. If the arc is narrowed down, then the current density increases. Naturally, the current density at the cathode tip also increases. If the current density increases, then there is greater damage to the cathode. The temperature at the cathode tip also naturally increases. By making the cathode tip have a protruding shape, the temperature at the cathode tip is more prone to rising than with a conventional tapered shape or the like. In addition, through the effects of the filler substance, the cathode tip becomes hotter in a mercury lamp than in a xenon lamp.
The center of the arc is at a high temperature, and the heat here travels to the surroundings through thermal conduction and so on. If thermal conduction is good, then heat travels rapidly from the high-temperature region, and hence heat escapes to the surroundings. The region in which the arc is formed (the high-temperature region) thus becomes narrow. The arc thus looks more narrowed down with a gas such as Ar than with Xe.
However, the inside of the arc is in a plasma state in which the gas or mercury is ionized. A current flows due to the movement of electrons and ions within the plasma. The arc being narrowed down means that the path along which the current travels is narrow. The current density is thus higher with a gas such as Ar than with Xe.
If the current density rises, then a central region of yet higher temperature is produced. The cathode tip, which is centrally positioned, is thus subjected to a yet higher temperature, and hence the temperature of the cathode tip tends to increase.
As a result, evaporation of thorium (Th), which is the electron-emitting substance, occurs more fiercely with a mercury lamp than with a xenon lamp, and hence the thorium is used up. Moreover, contraction of the arc spot called spot mode (a state in which the arc spot becomes point-like) is more prone to occur than with a xenon lamp. That is, fluctuation in the irradiance is brought about.
Furthermore, with a mercury lamp, because the load on the cathode tip is heightened, i.e. because the temperature at the cathode tip is more prone to becoming high, deformation of the cathode tip has frequently been seen to be brought about.
It is thus an object of the present invention to provide a short arc type mercury lamp according to which desired electrons can be discharged, contraction and wandering of the arc spot of the arc are suppressed, the cathode shape is not prone to deforming even if the lamp is lit for a long time, and fluctuation in the illuminance is suppressed.
The electron-emitting substance thorium separates out from grain boundaries inside the cathode, and this thorium makes emission of electrons at the cathode tip possible at a lower temperature than with tungsten, and thus carries out an electron discharge operation. In the case that carburization has been carried out, a large amount of thorium flows from the carburized surface, and is supplied to the tip through surface diffusion and so on.
As a result of assiduous studies, the present inventors have ascertained that, in the case of a mercury lamp having a protruding part on the cathode, arc-wandering such as spot mode and deformation of the cathode tip occur in accordance with the state of grain boundaries within the protruding part, specifically the number of grain boundaries intersecting a straight line that passes through the approximate center of an arbitrary section in the radial direction of the protruding part.
Moreover, with a mercury lamp having a cathode with a protruding part at the tip, because the cathode temperature is more prone to becoming high than with a conventional mercury lamp having a cathode with no protruding part or a conventional xenon lamp, there have been cases in which deformation of the cathode occurs markedly. Upon investigating the reason for this, it was found that this occurs in the case that the number of grain boundaries within the protruding part is extremely high.
If the number of grain boundaries becomes high, then this equates to voids having enlarged, which corresponds to thermal conduction having become worse. The flow of energy from the cathode tip to the cathode body is thus obstructed, and hence an abnormal temperature rise occurs at the tip. It is conjectured that the cathode tip undergoes deformation as a result.
To solve the above problems, an invention claimed in claim 1 is a short arc type mercury lamp in which a cathode and an anode are disposed opposite one another inside an arc tube, at least a noble gas and mercury are filled into the arc tube, a cone-shaped part that contains thorium oxide and continues on from the cathode body is formed on the cathode, and a protruding part that continues on from the cone-shaped part is formed; wherein the number of grain boundaries intersecting a straight line that passes through the approximate center of an arbitrary section in the radial direction of the protruding part is at least 0.5 per mm but not more than 100 per mm.
Moreover, an invention disclosed in claim 2 is a short arc type mercury lamp in which a cathode and an anode are disposed opposite one another inside an arc tube, at least a noble gas and mercury are filled into the arc tube, a cone-shaped part that contains thorium oxide and continues on from the cathode body is formed on the cathode, a protruding part that continues on from the cone-shaped part is formed, and at least part of the cone-shaped part and the cathode body is carburized; wherein the number of grain boundaries intersecting a straight line that passes through the approximate center of an arbitrary section in the radial direction of the protruding part is not more than 100 per mm.
By stipulating the number of grain boundaries intersecting a straight line that passes through the approximate center of an arbitrary section in the radial direction of the protruding part at the cathode tip to be at least 0.5 per unit length of 1 mm, it is possible to produce a discharge lamp for which arc-wandering is suppressed.
Moreover, by stipulating the number of grain boundaries intersecting a straight line that passes through the approximate center of an arbitrary section in the radial direction of the protruding part at the cathode tip to be not more than 100 per unit length of 1 mm, it is possible to provide a discharge lamp for which arc-wandering is suppressed, and there is little deformation of the cathode, even in the case of a mercury lamp having a protruding part at the cathode tip whose cathode temperature is more prone to becoming high.
Under the above conditions, with an aim of supplying thorium smoothly, if the side of the cone-shaped part and so on is carbonized, then it becomes easy to supply a sufficient amount of thorium from the cathode side to the cathode tip. By carrying out carbonization, the supply is possible. Emission of thermoelectrons is possible at a lower temperature with thorium than with tungsten. Consequently, emission of thermoelectrons becomes possible over a broader range than in the case that carbonization is not carried out, and hence the load on the cathode can be reduced. Proper cathode operation thus becomes possible even in the case that the number of grain boundaries intersecting a straight line that passes through the approximate center of an arbitrary section in the radial direction of the protruding part at the cathode tip is 0.5 or less per unit length of 1 mm, which led to arc-wandering in the case that carburization is not carried out.